DE4138157A1 - Measuring thickness of coating, e.g. of zinc@ on steel, - counting repeatedly applied laser pulses until spectral lines in plasma generated changes - Google Patents

Abstract

The measurement method involves repeatedly applying a focussed laser pulse of defined duration and intensity to one point on the surface of a workpiece. The material vaporised from the surface, whose quantity corresponds to the pulse duration and intensity, is analysed to determine the coating thickness. The spectrum of the plasma produced is analysed for spectral lines and bands characteristic of elements and/or molecules whose conc. values are determined. The number of pulses is counted until a detectable change in the spectral lines or bands occurs. The coating thickness is derived from the number and intensity of the pulses. ADVANTAGE - Accurate results obtained even on inaccessible surfaces.

Description

The invention relates to a method for determining the thickness of one applied to a base body Coating that's another Material composition has as the Basic body, using one on the Surface of the workpiece, focused laser pulse (shot), one of his Duration and intensity corresponding amount Material of the coating or the base body evaporates with the thickness of the coating Using an analysis of the evaporated material is determined.

The other material composition mentioned is of course given when completely different materials are available, e.g. B. a Base body made of steel and a coating Zinc or a body made of tool steel and a ceramic coating. Another Material composition in the sense of the invention but also given if two (or more) Elements or connections only in others percentage composition in the coating and are present in the basic body.  

From the book "Laser-Induced Plasmas and Applications "by L. J. Radziemski and D. A. Cremers, New York, Basel, Marcel Dekker Inc., 1989, S. 353/354, is a generic method known in which a laser pulse (shot) the Coating evaporates at the point in question and the evaporated material into a mixing chamber Introduction to an AAS system (flame atom Absorption spectroscopy system) becomes. The AAS signal is in relation to signals known layer thicknesses of the same Composition set and the layer thickness above Comparative values determined.

A disadvantage of this method is that the material evaporated from the laser pulse Mixing chamber must be supplied and that a Suction device up to the measuring point must be introduced, what is difficult to access Provide - if at all possible - with considerable difficulties. in the the rest is an indirect one Procedure, and the results obtained through an aerosol can be determined with the vaporized material must have errors.

DE-OS 40 04 627 describes a method for Determination of material properties of polymer Materials using a pulsed, on the surface of the focused laser beam is known. Each laser pulse generates a plasma with one for the elements contained in the material or Molecules characteristic radiation, this - Spectrally shifted in time in a spectral unit disassembled - in the form of spectral lines or Molecular bands detected by a detector unit  and from the radiation intensities selected elements / molecules based on numerical Ratio values the associated concentration values be determined. This is a Procedure in which the concentration is selected Material components of polymeric materials and their distribution over the surface determined will. A note, laser pulses for determination the thickness of a coating includes this font is not.

The invention is based on the object To specify the procedure of the type mentioned at the outset also performed in hard-to-reach places can be and the more accurate measurement results enables.

This task is first solved in that the laser pulse (shot) acting on the surface intervals is repeated that the Radiation from each individual laser pulse generated plasmas with those for those in the plasma contained elements and / or molecules characteristic spectral lines and / or Molecular bands in a spectral unit spectrally decomposed and the Spectral lines and / or molecular bands of one Detector unit fed and detected by this and that from the radiation intensities selected elements / molecules based on numerical Ratio values the associated concentration values be determined. Between the focusing device of the laser beam or laser pulses and the Workpiece and between the workpiece and the Spectral unit is not an immediate apparatus Connection, just an optical one Connection necessary. This makes certain  Distances between the named parts possible and it can e.g. B. also a workpiece in one Vacuum chamber to be examined, if one optical window is present. Others too difficult to access places can be workpieces to be examined.

In addition, the basis of the invention Task solved on the one hand in that the in repeated laser pulse intervals (pulsed laser beam) each on the same Point of the workpiece is directed, the Laser energy per laser pulse is kept constant and that the number of laser pulses counted as long until there is a noticeable change in the Spectral lines or molecular bands of the selected Sets elements or molecules, the thickness from the number of laser pulses counted in this way and whose energy is determined using comparative values. Counting the laser pulses to a noticeable one Changes in the spectral lines or molecular bands can yourself in a simple manner and with little effort accomplish. This additional measure is suitable preferred to be punctiform Thickness determination for fixed workpieces.

On the other hand, that of the invention underlying task additionally solved by that the repeated at intervals Laser pulse to a different location on the Material is directed, the each compared to individual laser pulse energy that of the previous laser pulse steadily is changed and that the energy of the laser pulse is determined in which there is a noticeable change the spectral lines or molecular bands of the selected elements or molecules,  where the thickness is from the energy so determined is determined via comparison values. These additional measure is preferably suitable for Determination of layer thickness of moving workpieces, such as B. tape strips. After receiving a measurement the energy of the laser beam is again from one lower or upper limit or initial value continuously changed.

With both additional measures, the Thickness of several layers on top of each other to be determined when the additional measures until another noticeable change in Spectral lines or molecular bands continued will. The thickness of the second, third, etc. Layer can then be used in the case of the first additional measure from the number of laser pulses between the noticeable changes and in the case of second additional measure - as a total thickness of all layers examined - from the energy of the Determine the laser pulse in which the second or third etc. noticeable change occurs.

By a corresponding variety of the at first additional measure on the measuring point directed laser pulses or changes in each directed to different places Laser pulses according to the second additional measure it is possible to get measurement results in given To get tolerances. The measuring accuracy leaves increase in that - in the case of the first additional measure - the energy per laser pulse or - in the case of the second additional measure - the change in energy by two each successive laser pulses kept small becomes.  

Advantageous developments of the invention are in described the subclaims. This is how - in the event the second additional measure - that everyone Laser pulse assigned energy compared to each previous laser pulses enlarged. If additionally the change (enlargement) of everyone Laser pulse associated energy with reaching the noticeable change in the spectral lines or Molecular bands set and from an upper or the lower initial value is changed (increased), will be a minimum of the material of the investigating workpiece by evaporation worn away.

Around the pulsed laser beam - also in the case the second additional measure - in simpler Way to a different location on the workpiece the workpiece is preferred continuously relative to the pulsed laser beam emotional. That can e.g. B. happen that a coated onto a coil or bundle Band uniformly under a pulsed laser beam is moved.

To such a band not only parallel to your own Longitudinal extension, but also in terms of area on the Examining the coating thickness, the pulsed laser beam additionally in one direction, the perpendicular to the relative movement of the workpiece runs vertically or at least one component has moved.

To difficult-to-reach places on workpieces It is still possible to achieve better provided the pulsed laser beams in the case both additional measures before focusing and / or the radiation emanating from the plasma  to the detector unit through an optical fiber respectively.

Embodiments of the invention are in the Drawing shown and will be described in more detail below explained. It shows

Fig. 1 shows the effect of the method on the basis of a basic construction of a geigneten for performing the process arrangement in the form of a diagrammatic sketch,

Fig. 2 is a spectrum for befindliches in a coating of pure titanium,

Fig. 3 is a spectrum for building under basic body of pure tungsten,

Fig. 4 is a diagram with the measured thickness of the titanium coating on the indexable insert along the line between points 5 'and 5 ''in Fig. 1 and

Fig. 5 shows the application of the method in a galvanized stainless steel strip.

In the exemplary embodiment shown in FIG. 1, the thickness of the coating made of titanium of an indexable insert 1 made of tungsten is determined.

A laser beam 3, which is pulsed at equal time intervals, is emitted from a laser unit 2 and directed by a deflecting mirror 4 onto a measuring point 5 . The deflecting mirror 4 is concave so that the laser beam 3 is focused between the deflecting mirror 4 and the indexable insert 1 . The focused part of the laser beam 3 is identified by the reference symbol 3 and is referred to only briefly below as the focused laser beam.

The action of the focused laser beam 3 'creates a hot, brightly glowing plasma 6 in the measuring point 5 . This plasma generates radiation 7 which is characteristic of the elements or molecules contained therein and which is fed to a spectrograph 8 . In this the plasma radiation is spectrally broken down by means of a grating 9 and in the form of individual spectral lines or possibly in the form of molecular bands 7 ', 7 '',. . ., 7 ⁿ detected with a time delay by a detector unit 10 designed as a diode row. The spectrum digitized there is then forwarded to a computer 11 which is used for storage and evaluation.

To several measuring points 5 ', 5 '',. . . to detect, the indexable insert 1 and the focused laser beam 3 'are moved relative to each other. This is accomplished in that the deflecting mirror 4 is pivoted iteratively about an axis 12 . Between the two outer measuring points 5 ', 5 ''reached by the pivoting of the deflecting mirror 4 , any number of measuring points can be detected on a line passing through these points.

The deflecting mirror 4 remains at rest at each measuring point. The laser unit 2 sends focused laser pulses 3 'at the same time from each the same energy. Each laser pulse 3 'vaporizes a small amount of the material it meets, corresponding to its energy, in addition to what the laser pulses generated in front of it have vaporized. As the number of laser pulses increases, a small hole-like recess is created in the insert.

First, the material of the titanium coating of the indexable insert 1 is evaporated and the one received by the detector unit 10 , in spectral lines 7 ', 7 ''. . . decomposed steel 7 generates a spectrum according to FIG. 2, which is forwarded by the detector unit 10 in digitized form to the computer 11 for storage and evaluation. Such detection is carried out separately for each laser pulse.

If one of the laser pulses 3 'hits the material of the base body of the indexable insert 1 made of tungsten after the material of the coating has completely evaporated at the measuring point 5 ⁿ concerned, then a spectrum according to FIG. 3 results.

The transition from the spectrum according to FIG. 2 to the spectrum according to FIG. 3 defines the transition from the coating to the base body in the case of the indexable insert 1 made of tungsten with a coating made of titanium, the number of laser pulses to the transition and their energy as a measure be evaluated for the thickness of the coating. The relation necessary for this evaluation can be z. B. using microscopic layer thickness comparison measurements.

The transition from one spectrum to another, in the present exemplary embodiment from the spectrum according to FIG. 2 to that according to FIG. 3, can be recognized by the significant change in certain line relationships of generally selected elements. This transition can be recognized on the screen and / or can be determined by a comparison with a predefinable setpoint in the computer.

In Fig. 4 is the thickness of the tungsten coating of the indexable insert 1 at the corresponding measuring points 5 , 5 ', 5 '', determined in the manner described with an approximate 3.3 / µm thickness. . . applied.

In order to be able to detect further measuring points on the indexable insert 1 , this is moved perpendicular to the line given by the measuring points 5 ', 5 ''relative to the focused laser beam 3 '.

In the exemplary embodiment shown in FIG. 5, the thickness of a zinc layer is measured on a sheet steel strip 12 . The belt 12 is guided continuously, coming from below, over a deflection roller 13 and passed on horizontally. Because of an obstacle 14 , the pulsed laser beam 3 emerging at a laser unit 2 'is passed through a light-wave lighter 15 and, at the end thereof, is focused on a measuring point 17 by an optical device 16 . each laser pulse of the focused laser beam 3 'evaporates - depending on the intensity - a small amount of material from the zinc layer or from the zinc layer and the steel sheet as the base material. In addition, as already described, a hot, brightly glowing plasma 18 arises in or immediately above the measuring point 17 . The radiation 19 emanating from this is again fed to a spectrograph 8 'with a grating and detector because of an obstacle (not shown this time) through an optical waveguide 20 . The received from the detector unit of the spectrograph 8 ', broken down into spectral lines radiation 19 generates a spectrum which is forwarded by the detector unit in digital form to a computer 11 ' for storage and evaluation.

With the pulsed laser beam 3 , the energy is changed from laser pulse to laser pulse. From a predefinable initial value, it is increased with each laser pulse. On the upper side of the band 12 , further previous measuring points near the edge are shown in FIG. 5. In the first laser pulses, an amount of zinc proportional to their energy evaporates, the penetration depth at the measuring points 17 increasing at the same time. In all cases, the spectrum specific to zinc results. If the energy is so great that steel (selected elements: Fe, C) also evaporates at a measuring point in addition to the zinc of the coating, the spectrum changes significantly. In this variant of the method, the energy of the laser pulse (shot) is used as a measure of the thickness of the zinc layer, in which some steel has already evaporated.

After achieving such a noticeable change in The spectrum is assigned to the laser pulses Energy again from a lower definable Initial value ramped up.

Claims (8)

1. Method for determining the thickness of a coating applied to a base body, which has a different material composition than the base body, using a focused laser pulse (shot) acting on the surface of the workpiece, which has an amount of material corresponding to its duration and intensity Coating or the base body evaporates, the thickness of the coating being determined with the aid of an analysis of the evaporated material, characterized in that
that the laser pulse ( 3 ') (shot) acting on the surface is repeated at time intervals and each directed to the same location ( 5, 5 ',...) of the workpiece ( 1 ), the laser energy per laser pulse ( 3 ' ) is kept constant,
that the radiation ( 7 ) of the plasma ( 6 ') generated by each individual laser pulse ( 3 ) with the spectral lines and / or molecular bands characteristic of the elements and / or molecules contained in the plasma ( 6 ) in a spectral unit ( 8 , 9 ) spectrally decomposed with a time delay and the spectral lines and / or molecular bands are fed to a detector unit ( 10 ) and detected by the latter,
that the associated concentration values are determined from the radiation intensities of selected elements / molecules on the basis of numerical ratio values,
and that the number of laser pulses is counted until there is a noticeable change in the spectral lines or molecular bands of the selected elements or molecules, the thickness of the coating being determined from the number of laser pulses counted in this way and their energy using comparative values. 2. Method for determining the thickness of a coating applied to a base body, which has a different material composition than the base body, using a briefly acting, focused laser pulse (shot) on the surface of the workpiece, the amount of material corresponding to its duration and intensity the coating or the base body evaporates, the thickness of the coating being determined with the aid of an analysis of the evaporated material, characterized in that
that the laser pulse acting on the surface ( 3 ') (shot) is repeated at intervals and in each case directed to a different location ( 17 ) of the workpiece ( 12 ), the energy associated with each laser pulse ( 3 ') being constant compared to the previous laser pulse is changed
that the radiation ( 19 ) of the plasma ( 18 ) generated by each individual laser pulse with the spectral lines and / or molecular bands characteristic of the elements and / or molecules contained in the plasma is spectrally separated in a spectral unit ( 8 ') and the spectral lines and / or molecular bands are fed to a detector unit and detected by it,
that the associated concentration values are determined from the radiation intensities of selected elements / molecules on the basis of numerical ratio values
and that the energy of the laser pulse ( 3 ') is determined, in which there is a noticeable change in the spectral lines or molecular bands of the selected elements or molecules, the thickness of the coating being determined from the energy determined in this way using comparison values. 3. The method according to claim 2, characterized in that the associated with each laser pulse ( 3 ') energy is increased compared to the previous laser pulse. 4. The method according to claim 2 or 3, characterized characterized that the change of everyone Laser pulse associated energy with reaching the noticeable change in the spectral lines or Molecular bands of the selected molecules or Molecules set and new from an initial value is started. 5. The method according to any one of claims 2 to 4, characterized in that the workpiece ( 12 ) is continuously moved relative to the pulsed laser beam ( 3 '). 6. The method according to claim 5, characterized in that the pulsed laser beam ( 3 ') is moved in a direction perpendicular to the relative movement of the workpiece ( 12 ) or at least has a vertical component thereto. 7. The method according to any one of the preceding claims, characterized in that the pulsed laser beam ( 3 ') is guided through an optical waveguide ( 15 ) before focusing. 8. Device according to one of the preceding claims, characterized in that the radiation ( 19 ) emanating from the plasma ( 18 ) is guided through an optical waveguide ( 20 ) to the spectrograph ( 8 ').